As a conventional apparatus including a voltage step-up/down converter, which steps up and down a DC voltage, an electric power steering apparatus is disclosed in JP 2003-267247 A. This electric power steering apparatus includes a voltage step-up/down circuit. The voltage step-up/down circuit is configured with an inductor, a step-up switch, a step-down switch and a step-up/down drive circuit. In the voltage step-up operation, the step-up/down drive circuit turns on and off the step-up switch to step-up (raise) a DC voltage outputted from a battery and supply the stepped-up voltage to a motor drive circuit. In the voltage step-down operation, the step-up/down drive circuit turns on and off the step-down switch to step-down (lower) a DC voltage outputted from the motor drive circuit and supply the stepped-down voltage to the battery.
If the step-down switch is short-circuited in the step-up/down circuit, the voltage step-up operation cannot be performed properly. It is not possible to detect short-circuit failure of the step-down switch.